1. Field of the Invention
This invention relates to improvements in mirrors having a reflecting surface provided by a metallized reflective sheet material, such as aluminized polyester, in a stretched condition over a supporting frame.
2. Description of the Prior Art
It has been generally known that lightweight planar mirrors can be produced by stretching over a generally peripheral ridge on a supporting frame, a thin flexible sheet material having a reflective surface. Examples of different constructions of such mirrors are found in various patents. For example, British Pat. No. 841, 875 to General Electric Company Limited, London, relates to a reflector having a reflecting polymeric material attached to a base material. Other patents include U.S. Pat. No. 3,180,220 to Jeffree; U.S. Pat. No. 3,434,181 to Benzies; U.S. Pat. No. 3,552,835 to Benzies; U.S. Pat. No. 3,608,179 to Jeffree et al; and U.S. Pat. No. 3,687,524 to Martinez.
One of the most important features in the construction of these mirrors is to take extraordinary care that the frames are flat and true or else the stretched sheet material will give a distorted image. The portion of the frame over which the reflective sheet material is stretched must be extremely flat, coplanar, and continuous. Even minute irregularities in the upper surface portion of the generally peripheral ridge over which the sheet material is stretched results in magnified distortions in the mirror surface with a loss of optical accuracy. Minor inaccuracies in the frame construction or deflections in the frame due to compressive and bending forces caused by the stretched condition of the film results in great distortions in the mirror surface. For example, deviations up to 0.001 inch in coplanarity of the portion of the frame over which the film is stretched were found to be detrimental to the mirror quality.
In the past, the preferred construction of mirrors of this type has included extruded aluminum, generally inwardly open channel members defining a frame. The channel members surround and hold a lightweight reinforcing substrate member between upper and lower flange members of the channel. The frame is bent into the desired shape with the end portions fastened together, or it is mitered and welded at its corners with great care taken to obtain a true planar frame so that when the reflective film is stretched thereover, it is as flat as possible so that the optical correctness required to produce a relatively distortion-free image is realized.
Once the film is stretched over the bead and placed in tension, it produces severe bending and twisting forces on the frame. It is partially the function of the reinforcing substrate to alleviate these forces and distribute them as uniformly as possible along the frame. The substrate alone however, which generally has been of a rigid foam plastic material, does not provide the requisite reinforcements and does not facilitate production of an optically correct mirror. U.S. Pat. No. 3,733,116 to Hutchinson discloses a means of avoiding the difficulties of the prior art by introducing a resilient member between a flange of the channel members of the frame and the supporting substrate which -- being in compression therebetween when the thin-film reflective sheet material is shrunk -- provides a reinforcement in the frame and resulted in an improved optically correct mirror.
Notwithstanding the improvement of Hutchinson the disadvantages of the prior mirrors continue to be existent. Primarily because of their complexity and the meticulous care required in their production, frames are expensive to produce. Backing substrate members involve still additional expenses. Excessive handling often results in damaged optical surfaces. Gripping capability of the film holding means -- usually an adhesive material, which, after curing, is quite rigid -- is often not sufficient to develop the maximum film tension possible with the most uniform stress distribution. Also the relatively rigid adhesives utilized permit variations in film tension from point to point around the periphery of the mirror due to temperature and humidity changes while "creeping" of the bond often results in deterioration of the optical qualities of the mirror surface. Moreover adhesively securing the film to the rearward and/or side surfaces of the frames having multiple components did not readily facilitate pressure adhesive securing techniques.
We have invented a mirror in which an optically correct and accurate mirror surface is provided by a metallized thin-film reflective sheet material maintained in a stretched condition over a unitary frame having a raised peripheral ridge or bead. Moreover, we have developed a new and improved method of producing such mirrors, as well as a unique apparatus particularly for practicing the method, wherein the disadvantages of the prior art mirrors are substantially eliminated, and the optical mirror qualities are substantially increased.